Erectus experiment: fossil find expands Stone Age anatomy.

During the heart of the Stone Age, from 1.7 million to 400,000 years ago, populations of our ancient ancestors in Africa, Asia, and Europe often served as brief evolutionary experiments, with most dying out before they established themselves as truly distinct species.

At least that's the implication of a peculiar fossil skull unearthed in eastern Africa last summer, according to its discoverers. The roughly 930,000-year-old cranium exhibits some features of Homo erectus as well as unique traits, say anthropologist Richard Potts of the Smithsonian Institution in Washington, D.C., and his colleagues. They describe the new find in the July 2 Science.

Stone Age specimens possessing the full anatomical signature of H. erectus exist only in China and Indonesia, in Potts' view. Variations on that skeletal theme at the African site and elsewhere arose from "short evolutionary experiments in small and fairly isolated populations that may have gone extinct as incipient species," he argues. An incipient species is an isolated group of animals presumed to be in the early stages of evolving into a new species.

In contrast, many researchers suspect that three or more full-fledged species of human ancestors coexisted with H. erectus (SN: 5/3/03, p. 275).

The new specimen, assembled from 11 cranial pieces, was discovered at Kenya's Olorgesailie site. Olorgesailie has yielded many stone hand axes but few remains of human ancestors.

The skull's estimated age derives from its position below a previously dated layer of volcanic rock and above a soil laver containing evidence of a reversal in Earth's magnetic field known to take place more than 700,000 years ago.

The Olorgesailie fossil displays some features of typical H. erectus crania, which are long and thick walled. However, even if the fossil turns out to be from a female, it's an unusually small skull for a human ancestor from that time, Potts says. The specimen's cranial capacity is considerably smaller than that of most H. erectus finds.

The new fossil exhibits other curious traits, such as a thin ridge of bone above the eye sockets rather than the pronounced bony crest associated with H. erectus.

Considerable body-size differences must have characterized Olorgesailie's ancient inhabitants, Potts holds. Only adults much larger than the newly discovered fossil individual could have struck pieces of rock from nearby outcrops to make tools. Skeletal development may have diverged in various ways for small and large individuals, contributing to the population's anatomical diversity, Potts theorizes.

Anthropologist Jeffrey H. Schwartz of the University of Pittsburgh welcomes the new find but still holds that a wide range of ancestral human species existed during the Stone Age. He regards only skulls previously found at two sites on Java as H. erectus. Other fossils found in Asia and Africa and sometimes attributed to H. erectus actually fall into two other anatomical groups that probably represent separate species, he holds.

The Olorgesailie skull's evolutionary identity remains unknown, Schwartz adds.